A more solid answer would emerge if, for instance, the molecular networks behind the formation of the appendix in monotremes and humans were compared, and found to use the same toolkit of genes — then we'd have to regard it as highly probable that they are homologous, and the last common ancestor had an appendix.

Bradford seems to think that modern evolutionary theory says that there “should” be some obvious event like the K-T impact to explain “major” events like the divergence of monotremes and marsupials+placentals, the divergence of marsupials from placentals, the divergence of the mammalian orders, the divergence of the mammalian families, etc.